I am looking for probes for determining the amount of ammonia, nitrite, and nitrate present in a liquid. Specifically, I'm looking for something I can use to monitor (not adjust, just monitor) the chemical status of my fish tank. Googleing hasn't found me anything but the MQ-137 which is a gas sensor for ammonia, not liquid.

I don't know if this will help or not but a couple of years ago I saw a project (on a website) that used light spectrum absorption to identify many different liquids. It used colored leds as emitters and sensors. If you could identify what if any color nitrates would absorb then you'd have it.

but I wonder what concentrations can be detected - accuracy / precision / ...

Here's a guy to ask!http://creative-technology.net/MAKE.html

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The spectrometer has a "learn" mode and an "identify" mode. In the learn mode a known sample is placed in the unit and sampled at each wavelength of light emitted by the LED's. The sampled values are stored in a database in the Arduino memory. In the identify mode an unknown sample is spectrally scanned and the software in the Arduino compares the values of the scan with the values stored in the database